A method for analysis by producing a mass spectrum by mass separation in a magnetic sector field of a mass spectrometer utilizing ionization of a sample substance by electron bombardment

ABSTRACT

In an analysis of a sample substance by mass spectroscopy, the sample substance is bombarded in a high vacuum chamber by an electron beam to produce ionized mass particles. To minimize ion residence time in the high vacuum chamber, the bombarding electron beam is interrupted at intervals to permit the ionized mass particles to be quickly drawn out of the high vacuum chamber by a suction voltage.

United States Patent Brunnee et a1.

METHOD FOR ANALYSIS BY PRODUCING A MASS SPECTRUM BY MASS SEPARATION IN AMAGNETIC SECTOR FIELD OF A MASS SPECTROMETER UTILIZING IONIZATION OF ASAMPLE SUBSTANCE BY ELECTRON BOMBARDMENT Inventors: Curt Brunnee, 282Platjenwerbe uber Vegesach, Birkenweg; Hans-Joachim Bultemann, WalliserStrasse 94, Bremen, both of Germany Filed: Apr. 14, 1971 Appl. No.:133,823

Related US. Application Data Continuation-in-part of Ser. No. 817,123,April 17, 1969, abandoned.

US. Cl. 250/413 SB, 250/419 ME Int. Cl. H0lj 37/08 Field of Search..250/41.9 ME, 41.9 G,

[451 Jan. 22, 1974 [56] References Cited UNITED STATES PATENTS 2,810,07510/1957 Hall 250/419 D 2,694,151 1l/1954 Berry 250/419 ME OTHERPUBLICATIONS Ionization In A Mass Spectrometer By MonoenergeticElectrons, A. E. Fox, Review of Scientific Instr., Vol. 26, No. 12, Dec.1955, pp. 1101-1107.

Primary Examiner.lames W. Lawrence Assistant Examiner-C. E. ChurchAttorney, Agent, or FirmWo1f, Greenfield & Sacks [5 7 ABSTRACT In ananalysis of a sample substance by mass spectroscopy, the samplesubstance is bombarded in a .high vacuum chamber by an electron beam toproduce ionized mass particles. To minimize ion residence time in thehigh vacuum chamber, the bombarding electron beam is interrupted atintervals to permit the ionized mass particles to be quickly drawn outof the high vacuum chamber by a suction voltage.

1 Claim, 7 Drawing Figures PATENTED 3.787. 681

SHEET 2 or 3 GRADIENT FROM SUCTION VOLTAOE LOW ELECTRON CURRENT NOTROUOII NO CHANOE OF ORAOIENT I /y/i LARGE ELECTRON CURRENT CHANGE OFORAOIENT t=O STATIONARY STATE WITH ION CURRENT AND RESIDENCE TIME Wg/ACMW QWWWXMQ v PMENTED JAN 2 2 ISM SHEET 3 BF 3 METHOD FOR ANALYSIS BYPRODUCING A MASS'SPECTRUM BY MASS SEPARATION IN A MAGNETIC sECTOR FIELDOF A MASS SPECTROMETER UTILIZING IONIZATION OF A SAMPLE SUBSTANCE BYELECTRON 5 BOMBARDMENT RELATED APPLICATION This application is acontinuation-in-part application of Ser. No. 817,123, filed Apr. 17,1969 and now abandoned.

FIELD OF THE INVENTION BACKGROUND OF THE INVENTION The known methods anddevices of this kind have the disadvantage that if the ion sourceoperates in a socalled space charge working condition, pressuredependent variations of the mass spectrum occur as a result of thepressure dependent ion residence time occuring in this operativecondition.

The space charge working condition is present under the following twocircumstances: l when a suction voltage is provided to draw ionizedparticles out of the ionization region; and (2) when an electronbombardment beam of an electron density is used which is so high that anegative space charge will occur which counteracts the suction voltageso that anion intercepting potential trough occurs in the ionizationregion.

In the space charge working condition the following quantitativerelation exists between the ion residence time t, and the operatingparameters of the ion source.

E 1 2E b e0V2e/mVUe 20 ll e. I 1) E0 /2 zo I 0 e 1 ln the above:

t, ion residence time m i,. ionizing electron current (A) b breadth ofthe electron bundle (cm) E potential gradient of the ion suction voltagein the ionization chamber, in the absence of a space charge W/ 2 U c c latiqn1ot sss of h isa zi s sls r n iv P gas pressure in the ionizationchamber Tim) s influence constant 8.86 by 1O (AsV" e charge of theelectrons m mass of the electrons 0' ionization probability of the gasesin the ionization chamber. Dependent on the kind of gas and the electronacceleration voltage U,,. Usual order of magnitude 1 10 (Torr If theconstants e, and VZe/m are inserted and if the usual values V and 9 Torrcm" are inserted for U, and 0' for normal operating conditions, thenthere is obtained [t 2 10 e/ N/1 e/ The space charge working conditionexists if i.e., with a potential gradient E of e.g., 5 VLcr r i /b mustbe greater than 5 "10*" A/ crF:

If e.g., i /b 1 10' A/cm and the pressure p 1 10 Torr, then the ionresidence time t, 10" s.

Under otherwise equal conditions, with a pressure of 10" Torr, the ionresidence time would be lO s.

lfi /b-E 10 AN, then the ion residence time is independent of theparameter i /b E and is only dependent on the pressure.

Equation (3) then becomes t, l0"/P'(s. Torr) The space charge condition,as will be seen from equation (1), is achieved if operation is done witha large electron current and a small ion suction current. In order toachieve a high detection sensitivity, as large as possible an electroncurrent has to be provided. However, the ion suction voltage cannot beincreased in the same ratio without producing ions of an increasedenergy spread that are unwanted and tend to reduce the resolution of thesystem. By these reasons operation in the space charge condition cannotbe avoided, if maximum sensitivity and resolution are to be obtained.

The ion residence time is particularly large with low pressures, e.g.,with residual gas measurements in the UHF region. As a result of this,the disturbing effects of the ion residence time become particularlystrong.

The present invention is directed to the problem of providing a methodand a device of the kind described above, which enables non-falsifiedgas analysis with high sensitivity, even in the pressure region below10' Torr.

In the solution of this problem, the invention is based on therecognition that the observed falsification of the mass spectrum withanalysis in ultra high vacuum is due to the unusually large ionresidence time in the ionization chamber. It is known, and is utilizedin mass spectrometry for investigating collision processes betweenelectrons and ions and between ions and neutral particles, that theenlargement of the ion residence time leads to formation of ionfractions and of multiply-charged ions by electron impact and byassociation between ions and molecules. Also, it is known that the ionresidence time in ultra high vacuum can be very long. From measurementsof the formation possibility, of multiply ionized noble gas ions, thisresidence time has been assessed at an order of magnitude of 100 ms(Redhead, P.A. I4Annual Conference on Mass Spectrometry and Allied Tops1966, pages 661-667).

These observations indicate that the falsification of the mass spectrumin ultra high vacuum can only be excluded if it is possible to avoid theexceptional elongation of the ion residence time in the ultra highvacuum. It is obvious to shorten the residence time by operating with ahigher ion suction voltage. Actually, with a sufficiently high ionsuction voltage, the pressure dependent alterations on the residual gasspectrum can be kept within bearable limits. The increasing of the ionsuction voltage acts unfavorably however, since the energy spread of theions is increased and thereby the resolution power is reduced. Thisdisadvantage can be minimized if the mass range is divided over two ioncollectors with different path radii. The voltage rise of the ionacceleration voltage is thus many times smaller than with theconventional arrangement with only one ion collector, so that even atthe upper end of the mass range, the ion acceleration voltage is stillso large that the relative energy spread which determines the resolutionpower, remains sufficiently small. On the other hand, the division ofthe mass range over two ion collectors is cumbersome and requires aspecial construction of the entire mass spectrometer, which cannot beprovided or can only be difficultly provided subsequently on existinginstallations.

In order to arrive at a better solution to the problem, the inventionproceeds from the consideration that the larger ion residence times inultra high vacuum are based on a negative space charge of the electroncurrent in the ionization region. This negative space charge produces,as is shown in FIG. I of the drawing, a potential trough which issuperimposed on the potential gradient produced by the ion suctionvoltage.

This potential trough becomes deeper, the smaller the potential gradientis in the ionization chambers. Ions are intercepted in the trough. Themagnitude of the thereby occurring ion space charge is so that thetrough just becomes flattened so far that the ions produced can flowaway. In the stationary condition, the ion space charge is given by theproduct of the ion current and the residence time. From this it can berecognized that, as explained above, by a sufficiently large potentialgradient the depth of the potential trough and thus the ion residencetime can be kept small (see FIG. 1). However, according to theinvention, independently of the magnitude of the potential gradientcaused in the ionization chamber by the suction' voltage, the ionresidence time can be kept practically as short as desired byperiodically interrupting the electron current, thus nullifying theelectron space charge and in consequence of this destroying thepotential trough. This can be realized by use of a pulsed electroncurrent source. The essence of the invention is directed to the use of apulsed ionizing electron current while operating in a space chargeworking condition of the ion source, i.e., when an ion interceptingpotential trough occurs in the ionization region. It is true that thesensitivity of the measurement is reduced in dependence on the pulsingratio, but this disadvantage can be compensated for by making theinterruption times for the electron current shorter than the currentflow times.

The method according to the invention can be used in a simple mannerwith existing mass spectrometers by providing a pulse device which isinsertable in the form of an adapter in the supply circuit of the ionsource.

By such a pulse operation of the electron current, falsifications of themass spectrum due to the residence effect can be practically completelyavoided in a simple manner, so that naturally true spectra can beobtained.

In order to make the invention clearly understood, reference will now bemade to the accompanying drawings which are given by way of example andin which:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates the troughsuperimposed on the potential gradient by the negative space chargewithin the ionization region.

FIGS. lA-ID illustrate potential gradients associated with differentelectron-ion conditions of the system, and helpful in understanding thetheory of the present invention;

FIG. 2 illustrates a mass spectrometer with a pulse device, according tothe invention, in a diagrammatic form; and

FIG. 3 illustrates a mass spectrum with unpulsed electron current withtwo different suction voltages, and the mass spectrum of the same gasmixture with pulsed electron current according to the invention.

DETAILED DESCRIPTION The drawing illustrates the use of the invention inmass spectrometers to produce a mass spectrum by mass separation in amagnetic sectoral field. Ion formation and ion separation take place ina chamber 1, which is kept at a pressure of preferably less than 10 Torrby a high vacuum pump 2. From a recipient 3, the gas sample to beanalyzed is passed into the chamber 1 through an inlet system not shownin the drawing. In the head la of the chamber, the sample substance isionized by a pulsed electron beam ionization source 4. The ions formedare drawn by a suction electrode 5 from the ionization region and areaccelerated by an ion optical system 6 consisting of a number ofelectrodes. The ions are then shot in the form of a beam or bundle 7through an entry slit 8 into the sectoral field of a magnet 9. In thesectoral field, a separation of the ions takes place by mass dependentdeflection, the magnitude of separation being variable by variation ofthe field strength of the magnet 9 or by variation of the accelerationvoltage of the ion optical system 6. The entire mass scale can be passedthrough ion collector 10, with an ion current measuring device 11connected thereto, at the outlet side of the magnetic field. Therefore,the various masses contained in the sample can be successively measuredat collector 10 by varying the magnetic field strength or theacceleration voltage.

In chamber 4, ionizing takes place by electron bombardment. For thispurpose, electrons from a thermionic cathode 12 are drawn into the entrywindow 13 of a metal box 14, which surrounds the ionization chamber 4,by a potential difference of e.g., volts. The electrons flow in the formof a narrowly limited beam through the chamber 4, emerge through awindow 16 of the chamber and pass to an electron collector 17. By meansof a magnet l8, 19 a magnetic field extending in the direction of theelectron beam 15 is produced. This field causes a sharp bundling of theelectron beam and thus causes a high electron density in the electronbeam.

The ions formed in the chamber 4 by electron bombardment are drawn outby a suction voltage U between the box 14 and the suction diaphragm 5closing the box at one side, of e.g., 5 volts, and are guided throughthe ion optical system in the already described manner and the analysisis performed by forming the mass spectrum.

A potential gradient is caused by the potential difference between thesuction diaphragm 5 and the rear wall of the box 14. This potentialgradient is shown in FIG. IA. This potential gradient is sufficient toinstantaneously draw out of the chamber 4 the ions formed by electronimpact, and the time which is necessary to withdraw the ions of highestmass number from the chamber 4, is in the order of micro-seconds. Thisprocedure is however disturbed by alterations of the potential gradient,which are produced by the space charge caused by the electrons in theionization chamber as is shown in FIG. 1C.

The problem solved by the present invention may be better understoodfrom a consideration of FIGS. lA-lD, which show the potential gradientbetween electrodes 14 and 5 of the ion source under different conditionsas set forthbelow. The electrodes I4 and 5 are depicted in FIG. 2. Theelectrode 5 is a suction electrode, whereas the electrode I4 forms partof the rear wall of the box shown in FIG. 2.

In FIG. 1A the straight line represents the potential gradient inabsence of the electron beam. When a beam of ionizing electrons iscaused to flow in the space between electrodes 14 and 5, the potentialgradient is altered as indicated in FIGS. 1B and 1C by the negativespace charge produced by the electrons. Now, if the electron current ishigh as is the case in FIG. 1C, a trough, in the negative direction, isformed in the line representing the potential gradient. The positivespace charge of the ions flattens this trough as much until thereversion of the potential gradient is abolished and per time unit asmany ions can flow off as are produced (FIG. ID). For producing thispositive space charge the ions must remain a certain time in thepotential trough. This residence time is inversely proportional to theion current. When the electron beam is interrupted, the negative spacecharge is removed, the potential gradient is then as indicated in FIG.IA, and the ions are instantaneously drawn out of the ionization regionby the voltage of electrode 5. If, however, the electron current issmall as is the case in FIG. 1B, no trough by gradient reversion butonly a small deviation from the straight line is formed and consequentlyno residence time of ions can result from the electron space charge.

FIG. 3A shows the mass spectrum of a gas sample using a suction voltageof 5 volts and a constant electron current of 1.3 mA. Relative to theunfalsified spectrogram, there is an increase in the proportion of ionfractions as a result of splitting of the molecular ions by electronimpact: e.g., the mass numbers 12 and 16 as fractions of the CO molecule(Mass number 28).

There is furthermore an increase in the proportion of multiply ionizedmass particles by repeated ionization of an ion by electron impact andmultiply charged ions occur already with electron energies which liebelow the ionization potential for direct multiple ionization: e.g.,occurrence of triply or quadruply ionized argon ions with highintensity. (Mass numbers 13 A; and 10).

Finally, the mass spectrum is falsified by the occurrence of associationproducts by collision between ions and molecules. There occurs e.g.,mass numbers 29 and 33 as association products of CO+ and H or 0 and H-The spectrum FIG. 3B was taken with an increased ion suction voltage of15 volts, and, shows a reduction of the disturbing effect caused by theion residence time, so that more particularly the fraction distributiondeviates less from the customary pattern.

In FIG. 3C, the mass spectrum is illustrated which is taken with asuction voltage of 5 volts and pulsed electron current on the order of 2milliamps.

For the pulsed operation, in the illustrated example a pulse generator22 is provided which with the aid of a switch 23 can be selectivelyconnected between the thermionic cathode 12 and the diaphragm 24provided between the thermionic cathode and the entry window 13 of theion source. The pulse generator produces a rectangular voltage havingthe pulse ratio 1 l, which is applied as a negative blocking voltage ofe.g., 50 volts relative to the cathode 12. This voltage is appliedperiodically to the diaphragm 24, and in turn the electron current inthe ion source is interrupted. For exam ple, triply and quadruplycharged ions (mass numbers 13 /3 and 10) and the association product(COH) (mass number 29) are no longer present.

The period of the current flow times and current interruption timeamounts to 50 microseconds. Since the ions are instantaneously drawnfrom the ionization zone on disappearance of the potential trough afterinterruption of the electron current, there exists an ion residence timeof 50 microseconds maximum. Because of the short residence time, theabove described disturbance effects cannot noticeably occur.

Within the scope of the invention, many modifications and otherembodiments are possible. More particularly, a pulse ratio for theblocking voltage U, of other than 1 i can be selected, so that theblocking time can be restricted to the smallest value possible in orderto limit the reduction in sensitivity caused by the pulsing.

Furthermore it is possible to make the duration of the electron currentperiods larger or smaller than 50 microseconds. According to experience,the electron current periods and thus the ion residence time should notbe greater than 1 ms.

If the ion residence time is smaller than the on time of the electroncurrent, a part of the ions can leave the ionization chamber alreadyduring the on time of the electron current. Since the startingpotentials of the ions which start during the on and off times aredifferent, there is an undesired peak separation. The pulsing frequencymust accordingly be selected high enough so that the on time of theelectron current is always smaller than the ion residence time given bythe operating parameters. If this cannot be achieved, then a changeovermust be made to continuous electron current.

be controlled.

The magnitude of the blocking voltage U, is adaptable to particularconditions. The blocking voltage should not be made any larger than; issufficient for counteracting the potential trough in the ionizationchamber, because otherwise a large part of the ions are drawn to thecathode chamber, which would result in a reduction of sensitivity. Thisaction can moreover be reduced if an additional diaphragm 26 lying at apositive potential is arranged between the diaphragm 24 and the box 14.

No requirements are placed on the frequency and amplitude constancy ofthe blocking voltage, since its values are uncritical. The operationwith increased suction voltage is only an emergency aid compared withpulse operation, since with increasing pressure and increasing electroncurrents the residence times without pulsed operation again becomeunbearably large. With the pulsed operation it is possible with largeelectron currents and small ion suction voltages, to overcome the spacecharge influences on the fraction spectrum completely, even with lowestpressures.

In the mass spectrometer of FIG. 2 the following potentials may beapplied:

suction electrode 5: 5 to l5 volts against 14 cathode 12: -70 to 100volts against 14 box 14: some 100 volts to some 1000 volts againstground electron collector l7: 0 to volts against 14 diaphragm 24: 0 voltagainst 12 during on time of switch 23 diaphragm 24: 30 to 50 voltsagainst 12 during off time of switch 23 The operation with pulsedelectron current in an ion source is known, but has hitherto been usedfor another purpose and with another result. Thus, in the so-calledFox-ion source (R. E. Fox et al in'Review Scientific Instruments 26(1955), 1101-1107) an alternating pulsing of the electron current andpusher voltage is used in order to prevent the energy of the ionizingelectrons from being influenced by the pusher voltage. The aim is thusthe production of an extremely monoenergetic beam of the ionizingelectrons. The purpose of the pulsing electron source in the presentinvention is to decrease the ion residence time, which time is notinherent in the Fox device by virtue of the absence of a negative spacecharge problem in the ionization region. Stated in another way, the Foxsource is designed to obtain ionization probability curves. To obtainexact values of the electron acceleration voltage, it is important toavoid producing a beam of multi-energetic electrons. As is pointed outin the Fox reference, the probability curves are measured withessentially monodence time, in the present ionization chamber. Theappearance potential defining the limit of ionization probability is afunction of the electron emission from the filament. Thus at increasedelectron currents, even those in the microamp region but generally above4 microamps, the space charge at the retarding slit changes the energydistribution across the slit space in such a manner that those electronspassing through the center of the slit will receive a different energythan those passing through the edge of the slit, so that the energy ofthe electrons passing the slit is no longer mono-energetic.

It should also be pointed out that the space charge that could occur isextremely small and does not produce any trough or gradient reversion asshown in FIG. 1B because the Fox source generally operates at anelectron current level a thousandfold less than the elec tron currentlevel in the present ion source which preferably is in the region of 1,3milliamperes.

On the other hand in the present invention an extremely mono-energeticelectron current is notnecessary and therefore the space charges at theentrance of the electron current into the ionization chamber,

, which are troublesome with respect to the purpose of the Fox ionsource, are not so with the present invention. While the small spacecharges within the ionization zone in the Fox device cannot cause an ionresiinvention, which uses a thousandfold higher electron current, theion residence time was a problem.

In the Bendix aviation mass spectrometer there is likewise analternating keying of the electron current and ion suction impulse witha delay time therebetween during which the spatial distribution of ionswith different commencement energy is so adjusted that with suitablesource parameters the influences of ion energy and ion starting point onthe passage time are compensated. Accordingly, an improvement of theresolution is obtained by compensation of the influences of thecommencement energy and the commencement region of the ions.

In contrast to this, in the present invention a pulsing of the electroncurrent has been used to overcome the pressure-dependent alterations ofthe mass spectrum, which occur in the space charge condition as a resultof the ion residence time. Thus, the method of the present invention isparticularly applicable to partial pressure analyzers, for example,wherein a high detection sensitivity is achieved operating with a largeelectron current.

energetic electrons. This is of course possible only at lower energylevels and moreover with only a very' What is claimed is:

l. A method for analysis with high detection sensitivity of a samplesubstance by producing a mass spec trum by mass separation in a magneticsector field of a mass spectrometer, comprising the steps of;

1. bombarding the sample substance in a high vacuum chamber with a highdensity electrom beam 'of a current greater than 0.5 miliamperes toproduce ionized mass particles, said electron beam characterized by anegative space charge potential 7 having an ion intercepting potentialtrough 2. intermittently interrupting the electron beam to reduce ionresidence time resulting from the ion intercepting potential trough,

3. providing a suction voltage at least during the interuption of theelectron beam to draw ionized mass particles out of the ionizationregion of the chamher,

4. and separating said ionized mass particles in the magnetic sectorfield.

1. A method for analysis with high detection sensitivity of a samplesubstance by producing a mass spectrum by mass separation in a magneticsector field of a mass spectrometer, comprising the steps of; 1.bombarding the sample substance in a high vacuum chamber with a highdensity electrom beam of a current greater than 0.5 miliamperes toproduce ionized mass particles, said electron beam characterized by anegative space charge potential having an ion intercepting potentialtrough
 2. intermittently interrupting the electron beam to reduce ionresidence time resulting from the ion intercepting potential trough, 3.providing a suction voltage at least during the interuption of theelectron beam to draw ionized mass particles out of the ionizationregion of the chamber,
 4. and separating said ionized mass particles inthe magnetic sector field.
 2. intermittently interrupting the electronbeam to reduce ion residence time resulting from the ion interceptingpotential trough,
 3. providing a suction voltage at least during theinteruption of the electron beam to draw ionized mass particles out ofthe ionization region of the chamber,
 4. and separating said ionizedmass particles in the magnetic sector field.